As is known, for example, from the EP-A-0 278 752, aluminum alloys can be tin-plated by the exchange method with acidic, halogen-containing additives containing tin salt electrolytes. All the halogen compounds named there are used clearly below their solubility in the electrolyte. This means that these compounds, during constant use of the electrolytes, must be analyzed and adjusted regularly (relatively expensive analyses), in order to keep their content within the necessary solubility ranges.
The present invention therefore is directed to the technical problem of finding halogen-containing additives, with which it is possible to keep the effective halogen content practically constant without the need for relatively expensive analyses and corresponding adjustments and thus to achieve a constant tin-plating quality without great expense.